A power supply is a power processing circuit that converts an input voltage or current source waveform into a specified output voltage or current waveform. A switched-mode power supply is a frequently employed power supply that converts an input voltage waveform into a specified output voltage or current waveform. There are several families of switched-mode power supplies employing a variety of topologies such as a boost topology.
In off-line power supply applications, a high power factor is frequently required. The power factor is defined as a ratio of the actual power delivered to the load to a product of the voltage and current at the input of the power supply. While a power factor of unity is the ultimate goal, a lesser power factor may, in some cases, be considered acceptable. Therefore, in many off-line applications, a power factor corrector (PFC) may be necessary to provide an acceptable power factor.
A power factor corrector employing a boost topology generally includes a boost inductor and a power switch coupled to the boost inductor. The power factor corrector further includes a rectifier (a rectifying diode) coupled to a node between the boost inductor and the power switch. The power factor corrector still further includes an output capacitor coupled across an output thereof. The output capacitor is usually large to ensure a constant output voltage. A load is then connected in parallel across the output capacitor.
The power factor corrector generally operates as follows. The power switch is closed (conducting) for a first interval D (D interval). The rectifying diode is reverse-biased, isolating the output capacitor and, therefore, the load from the input of the power factor corrector. During this interval, the input voltage supplies energy to charge the boost inductor and the inductor current increases. Since the load is isolated from the input voltage, a stored charge in the output capacitor powers the load. Then, for a second interval 1-D (1-D interval), the power switch is opened (non-conducting). The inductor current decreases as energy from both the boost inductor and the input flows forward through the rectifying diode to charge the output capacitor and power the load. By varying a duty cycle of the power switch, the output voltage may be controlled.
Conventional power factor correctors employing the boost converter topology, however, cannot directly process the AC power available from the AC line. An input full wave rectifier bridge is required at the input to rectify the AC voltage from the AC line. The rectified AC voltage may then be processed by the power factor corrector. The rectifier bridge is subject to dissipative losses, however, particularly at low AC line voltages (e.g., 85 to 100 VAC). Power dissipation in the bridge diodes of the rectifier bridge may be as high as 2 to 3% of the total power processed by the power supply. Further, the rectifier bridge may contribute to electromagnetic interference noise generated by the power supply.
Analogous to other types of power supplies, the power factor corrector is subject to inefficiencies that impair its overall performance. More specifically, the power switch and rectifying diode may be subject to conduction losses that reduce the efficiency of the power factor corrector. Additionally, the power switch [e.g., a metal-oxide semiconductor field-effect transistor (MOSFET)] is subject to switching losses that occur, in part, when a charge built up in a parasitic capacitance of the power switch is dissipated when the power switch is closed (turned ON). Furthermore, the rectifying diode may also be subject to a reverse recovery phenomenon, when the power switch is closed (turned ON), that induces a substantial current spike through both the power switch and the rectifying diode. The losses associated with the power switch and the rectifying diode increase linearly as the switching frequency of the power supply is increased. Therefore, the reverse recovery and switching losses associated with the rectifying diode and power switch will impair the overall efficiency of the power supply.
Accordingly, what is needed in the art is an active clamp, employable with the power factor corrector, that reduces the losses associated with the rectifier bridge and the reverse recovery phenomenon and further reduces the switching losses associated with the power switch(es) of the power factor corrector.